Polyurethane elastomers have become a commerically significant group of organic polymer materials. They generally stand out due to the good strength thereof, the combination of good hardness with good elongation at break and the normally good resistance to wear.
An exceptionally large number of structural variations in combination with an equally large variety of properties is possible using diverse low molecular weight and high molecular weight reactants as starting materials.
The casting process is conventionally used to produce polyurethane-polyurea elastomers from reactive starting compounds. There are two possible procedures to this end, which are distinguishable by the order in which the reaction components are added.
In the one-shot process, the components are all mixed simultaneously according to gravimetric or volumetric metering and then poured into molds. The chemical reactions therefore start simultaneously, but differ somewhat due to the varying reactivity thereof (or by corresponding catalysis). The reactions are exothermic. Since it is impossible in the one-shot process to remove the heat in stages, additional complications may easily occur. Typical of the problems are a lack of homogeneity in the casting due to varying temperatures between the peripheral phase and the core, the formation of bubbles or cracks by overheating in the core, insufficient binding of the hard and soft segments by pronounced differences in the reactivity of the individual components, and intensified secondary reactions, such as the formation of isocyanurate and carbodiimide.
An ordered chemical structure of the soft and hard segments cannot be expected with the one-shot process. Soft segment units having a varied number of isocyanate-linked relatively high molecular weight reactants and hard segment units having a varied number of isocyanate-linked chain-lengthening agents are produced.
For this reason, the so-called "prepolymer technique", is used in commercial polyurethane resin systems.
In almost every case, the long-chain diol component (polyester, polyether) is partially or completely reacted with the diisocyanate. An OH-- or NCO--terminated prepolymer is produced, depending on the equivalent ratio of the starting components. Oligomeric prepolymers having OH--end groups are generally unsuitable for casting processing due to the very high viscosity thereof, but do have limited use in urethane-rubbers. In comparison, many casting systems are based on prepolymers with most systems containing definite quantities of excess diisocyanates (semi-prepolymer, pre-adduct). This reaction product from the first reaction stage is either produced by the user himself, shortly before it is subsequently processed, or is delivered in this form by producers of raw materials. If this is the case, the product must have a storage stability of several months. Prepolymers obtained from exactly 2 mols of a diisocyanate and 1 mol of a long-chain diol are frequently used to cross-link bulky aromatic diamines. As opposed to semi-prepolymers, these prepolymers have the physiological advantage of not containing any volatile monomeric diisocyanates. This can be a concern from a health point of view when casting articles in open molds.
The NCO/OH ratio of the starting components is above 1 (up to about 1.15) in all casting systems. In other words several NCO groups have no OH functional material with which to react. These NCO groups react with the urethane groups which are formed and partially with atmospheric moisture causing chemical cross-linking.
The physical cross-linking (semi-crystalline hard segment association via the formation of hydrogen bonding) may be chemically completed by the formation of allophanate or biuret. In practice, chemical cross-linking takes place in the solid phase and is the reason why almost all glycol-cross-linked systems require subsequent annealing. This is necessary to achieve optimum properties in the material. Chemical and physical cross-linking together produce optimum material properties. In amine cross-linking systems, the hard segments segregate more rapidly due to the greater polarity of the urea groups. For this reason, amine-cross-linked products require a short period of subsequent heating. In amine-cross-linking, the processing temperature and the NCO-index has a qreater influence on the properties than in glycol-cross-linking.
Polyurethane/polyurea elastomers having an almost exact chemical structure of hard and soft segments may only be expected if prepolymers of 2 mols of a diisocyanate and 1 mol of a long-chain diol (polyether/polyester) are used and are reacted with 1 mol of a chain-lengthening agent (diol/aromatic diamine). In spite of the supposedly stoichiometric structure thereof, 2:1 NCO prepolymers of this type have a "Flory" distribution. In other words, they have corresponding quantities of NCO prepolymer molecules in which the long chain diols have been "preextended" by one or more diisocyanates, forming urethane bridges, as well as free diisocyanate molecules. If NCO prepolymer (mixtures) of this type are reacted with chain-lengthening agents, widened distributions of hard segments are again formed. Products of an unsatisfactory quality are obtained by this method with diols, so that diols among others, are reacted with so-called "semi-prepolymers", which contain a definite quantity of free diisocyanate in addition to the NCO prepolymer and thus enable a sufficient proportion of hard segments to be formed. If 2:1 prepolymers are used, aromatic diamines are preferably used as chain-lengthening agents or cross-linking agents.
The introduction and homogeneous distribution of the cross-linking agent into the prepolymer of a polyurethane casting system and the filling of the reaction composition into casting molds (which are almost always open and heated), must be done in two stages which directly follow each other since the available time (depending on the system) is at most only a few minutes and in some cases, is only a few seconds. The rapidly progressing reaction causes the viscosity to increase sharply and causes the composition to solidify quickly. The composition, in most cases, still requires subsequent heating to achieve its final physical properties.
In practice, the operations for stirring in the cross-linking agent and filling the molds are either carried out discontinuously in a manual process or else continuously-operating multi-component casting apparatus is used. It is also possible intermittently to process hot casting systems in conjunction with a shot-wise method for filling the mold.
The introduction and mixing of the cross-linking agent and the filling of the reaction composition into the casting mold is particularly troublesome when aromatic diamines are used, due to the inherent high reactivity thereof with respect to isocyanates. There have been many attempts to reduce the reactivity of the aromatic amines by appropriate modifications, such as by using amines substituted with bulky and/or extremely electron-withdrawing groups (German Offenlegungsschrift No. 3,012,864) or by using aromatic amine/salt complexes (U.S. Pat. No. 3,891,606). These processes suffer from disadvantages, e.g. the high cost of the aromatic diamines which are substituted by carboxylic acid ester or sulphonamide groups.
Processes are also known in which a finely-divided aromatic diamine is suspended in a polyhydroxyl compound and subsequently mixed with a polyisocyanate or an isocyanate group-containing prepolymer at a temperature below the melting point of the diamine. The composition may be cured at a temperature below the melting point of the diamine (German Offenlegungsschrift No. 2,635,400) or above the melting point of the diamine (German Auslegungsschrift No. 1,122,699). These so called "heterogeneous" processes (wherein diamines used as chain-lengthening agents may be reacted heterogeneously), permit longer processing times. However, doubts about the toxicity thereof still exist. Furthermore, the diamines are preferably used suspended in polyhydroxyl compounds so that a smooth-running reaction is difficult to achieve since the reaction velocities with respect to isocyanate groups differ markedly. Reaction injection molded elastomers on the basis of amine terminated polyethers are described in U.S. Pat. Nos. 4,444,910 and 4,443,067.